1. Field of the Invention
The present invention relates generally to a thin-film transistor array suitable for the manufacture of an image display utilizing liquid crystal, and more particularly to a gate electrode of the thin-film transistor array.
2. Description of the Prior Art
Of various thin-film displays currently available in the market, liquid crystal displays are highly evaluated because they consume a minimized amount of electric power and can readily be fabricated into a color version faithful to present full-color images. In these days, development of large-sized display screens is actively being pursued.
A thin-film transistor array (hereinafter referred to simply as a TFT array) employed in conventional liquid crystal displays generally comprises a glass substrate, a gate electrode made of, for example, aluminum formed on the glass substrate, and a first amorphous silicon semiconductor layer formed on the gate electrode via a silicon nitride gate insulating film. The TFT array also comprises a source electrode and a drain electrode both formed on the first amorphous silicon semiconductor layer via a second amorphous silicon semiconductor layer containing phosphorus, and a transparent display electrode formed on the gate insulating film in electrically connected fashion with the drain electrode and used for applying a voltage to liquid crystal. Each of the source electrode and the drain electrode is of a double-layered structure including a layer of titanium (Ti) and that of aluminum (Al). In the above-described construction, the surface of the gate electrode is occasionally covered with an Al oxide, thereby enhancing insulating properties.
The TFT array having the above-described construction is manufactured as follows. The glass substrate is initially covered with an Al film, which is in turn configured into the gate electrode by the use of a photolithography technique and by etching. In applications where the Al gate electrode is covered with an Al oxide, an anodization process is effectively utilized. The main materials of the TFT, i.e., the gate insulating film of a silicon nitride (SiNx), the first amorphous silicon (a-Si) semiconductor layer, and the second amorphous silicon (n.sup.+ -a-Si) semiconductor layer provided to obtain ohmic contact between the source or drain electrode and the first semiconductor layer are successively formed utilizing the CVD (chemical vapor deposition) method. A portion of the first (a-Si) semiconductor layer and that of the second (n.sup.+ -a-Si) semiconductor layer other than those portions thereof which do constitute the TFT are then removed by etching. Thereafter, an indium tin oxide (ITO) is formed on the gate insulating film and is configured into a transparent display electrode by the photolithography and by etching. Then, an opening is formed which extends through the gate insulating film to obtain electrical contact between the gate electrode and a source wiring. Thereafter, a Ti film and an Al film are formed in this order and are configured into the source and drain electrodes by the photolithography and by etching. Upon removing the second (n.sup.+ -a-Si) semiconductor layer from channel portions of the TFT, the TFT array is completed.
The above-described conventional TFT array requires a heating process, which is carried out at 130.degree. C. in the process of photolithography, in preparation for the subsequent etching required to form the gate electrode having a desired configuration, or requires a process in which the temperature of the substrate reaches 300.degree. C. during the formation of the silicon nitride gate insulating film or the first (a-Si) semiconductor layer. Because of this, it is likely that so-called hillocks (grown-up projections) of Al are produced. If the hillocks are produced, the Al oxide formed by the anodization process cannot fully cover the projections, thereby reducing insulating properties. As a result, the problem arises that short-circuiting takes place between the gate electrode and the source electrode, or another problem arises that the Al surface of a connecting portion between the gate electrode and the source wiring falls to ruin, thereby causing contact failure.
Conventionally, in order to prevent the generation of such Al hillocks, about 2 atomic % of silicon (Si) is occasionally added as an impurity to Al. As shown in FIG. 2, however, an Al oxide formed by anodizing Al containing Si as the impurity brings about a large leakage current, and the insulating properties between layers are fairly bad. When such Al oxide is employed as an insulating film, the problem arises that shortcircuiting frequently takes place between the gate electrode and the source or drain electrode.